In an article published in the Jan. 18, 2001 issue of the journal Nature, University of Iowa space physicist Donald Gurnett says that a search for lightning on Venus in 1998 and 1999 using the Cassini spacecraft failed to detect high-frequency radio waves commonly associated with lightning. Gurnett's paper is certain to be of interest to other space physicists, for whom the possible existence of lightning at Venus has long been controversial.
"If lightning exists in the Venusian atmosphere, it is either extremely rare, or very different from terrestrial lightning," Gurnett says. "If terrestrial-like lightning were occurring in the atmosphere of Venus within the region viewed by Cassini, it would have been easily detectable."
The Cassini spacecraft, which made its closest encounter with Jupiter on Dec. 30, 2000 and is scheduled to arrive at Saturn in July 2004, made two gravity-assisted fly-bys of Venus, the first on April 26, 1998 and the second on June 24, 1999. During the fly-bys the Radio and Plasma Wave Science Instrument (RPWS), with its three, 30-foot-long antennas, searched for impulsive high-frequency (0.125 to 16 MHz) radio signals. Gurnett, who serves as RPWS principal investigator, says that these signals, called "spherics," are always produced by lightning on Earth and are commonly heard as static on AM radios during thunderstorms. As a test of the RPWS ability to detect Earth-generated lightning, a search was conducted for spherics as Cassini made a close fly-by of the Earth on August 18, 1999. Not surprisingly, the instrument detected lightning continuously at rates up to 70 impulses per second while Cassini was located closer than 14 Earth radii.
Despite the Cassini results, Gurnett cannot rule out the possibility that some type of low-frequency electrical activity may yet exist at Venus because radio signals cannot penetrate the ionosphere at frequencies below about 1 MHz. Therefore, no definitive statement can be made about the lightning spectrum at frequencies below about 1 MHz.
"Since the atmosphere of Venus is very different from that of Earth, it is perhaps not surprising that electrical activity on Venus might be very different from lightning in the Earth's atmosphere," says Gurnett, who notes that lightning generally can be divided into two types, cloud-to-ground and the weaker cloud-to-cloud variety. "Because clouds over Venus are at very high altitudes of 40 kilometers or more, it is likely that lightning at Venus, if it exists, is primarily cloud-to-cloud. Terrestrial cloud-to-ground lightning is generally more intense than cloud-to-cloud so it is possible that the absence of impulsive high-frequency radio signals during the Venus fly-bys could be owing to the dominance of very weak cloud-to-cloud lightning at Venus."
Gurnett says that electrical activity at Venus could also be cloud-to-ionosphere discharges. "At the Earth, there is a type of electrical discharge called a "sprite" that travels up from a cloud to the ionosphere. A sprite is not like lightning as we usually think of it," Gurnett says. "Sprites have a slow electrical discharge, meaning that they also have a low frequency and are very difficult to detect."
Serious discussions over whether lightning exists at Venus began in 1978 when Venera, Russia's Venus lander, found low-frequency signals that some scientists called lightning, but others doubted for a variety of reasons. Later, physicist William Taylor, a former UI student of Gurnett's, in 1979 found what he considered to be evidence for lightning using the NASA Pioneer-Venus spacecraft. In 1990 using a Galileo spacecraft instrument similar to the one he designed for Cassini, Gurnett detected several small impulses that were interpreted at the time as being indicative of lightning. However, Galileo was some 60 times more distant from Venus than was Cassini, making the results much less significant than those of Cassini.
Meanwhile, the Cassini spacecraft, launched in 1997, is continuing its journey to Saturn, where it is scheduled to begin a four-year exploration of Saturn, its rings, atmosphere and moons on July 1, 2004. Under the terms of a $9.6 million NASA contract, Gurnett and an international team of some18 co-investigators will use the RPWS to measure Saturn's powerful radio emissions, as well as its lightning discharges.
Gurnett, a member of the National Academy of Sciences, is a veteran of more than 25 major spacecraft projects, including the Voyager 1 and Voyager 2 flights to the outer planets, the Galileo mission to Jupiter, and the Cassini mission to Saturn. He made the first observations of plasma waves and low-frequency radio emissions in the magnetospheres of Jupiter, Saturn, Uranus and Neptune and discovered lightning in the atmospheres of Jupiter and Neptune. Gurnett's University of Iowa co-authors in the Nature article include William Kurth, George Hospodarsky, and Terry Averkamp.
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